Solar tower thermal power generation technology has the advantages of high light-electricity conversion efficiency and flexible energy storage, and it has become one of the most important ways to achieve the goal of “carbon peak and carbon neutrality”. The heliostat is the core device to realize solar energy pooling in the whole tower photovoltaic power plant. It can focus the solar energy within a certain range around the heat-absorbing tower, so as to complete the light-heat conversion in the heat-absorbing device with high quality. However, the heliostat is susceptible to structural deformation due to self-weight and wind load during service, and the distance between the heliostat and the heat-absorbing tower is relatively large. The small deformation of the reflecting mirror surface of the heliostat will lead to the reflected solar beam falling on the heat-absorbing device, which will directly lead to a decrease in the light-concentrating efficiency of the heliostat and a deterioration in the quality of the light-energy distribution of the heat-absorbing device. Therefore, it is important to investigate the service optical accuracy of the heliostat under self-weight and wind load, which can provide a basis for its lightweight and high-precision design and service performance guarantee.

In this study, we study a typical lightweight small-scale heliostat of 20 m². First, we establishe the three-dimensional (3D) model and the finite element simulation model. Second, we adopte a universal optical-machine integration modeling method previously proposed by the team to consider the service deformation of solar concentrators and establishe an optical-mechanical integration analytical model of the heliostat. We also investigate the influence of the key structural parameters under the effect of self-weight and wind load on the optical accuracy of the heliostat in service. The structural parameters include the spacing d and the number N of the three rows of supporting bolts at the bottom of each plane mirror, the structural parameters of the frame supporting the mirrors, and the parameters of the beam cross-section. The conclusions of the study can provide an important basis for the design of lightweight high-precision heliostat and the maintenance of heliostat's performance in service.

When the wind pressure load on the mirror surface is equal, the self-load at a height angle of 90° has the most unfavorable influence on its optical accuracy. At N=7, the maximum deformation with height angle only increases from 2.07 mm to 2.63 mm, but D_x will significantly increase from 4.21 mrad to 5.35 mrad, and D_y will only increase from 1.36 mrad to 1.73 mrad (Fig. 3). Increasing the number of supporting bolts can reduce the concave deformation of the mirror surface between adjacent bolts in a single row, thereby reducing the slope error component D_y (Fig. 4). By comparing the distribution of mirror deformation and slope error, it can be clearly observed that the maximum slope error region does not occur in the maximum deformation region, which means that there is no non-linear positive correlation between mirror total slope error and mirror deformation (Fig. 5). The decrease in stiffness of the mirror itself is the main reason for the decrease in its optical accuracy, and a support rib plate structure should be added to the back of the mirror according to actual needs, so as to enhance its load-bearing stiffness (Fig. 6). Simply increasing the size of the cantilever secondary beam angle steel may not necessarily improve the optical accuracy of the mirror but rather result in material waste. To further improve optical accuracy, efforts should be made on the structural load-bearing stiffness of the mirror itself, such as adding a support plate on the back of the mirror (Figs. 7-8). The slope of the mirror total slope error variation curve at different height angles is basically the same, with only translation differences (Fig. 9).

If the frame structure is not deformed, the deformation and slope error of the mirror surface under the same wind pressure increase with the increase in the height angle of the heliostat mirror, and the slope error component D_x along the mirror surface in the transverse direction is significantly larger than D_y along its vertical direction; the increase in the number N of mirror back bolts does not have a significant effect on D_x, but it can significantly reduce D_y,and the improvement is no longer obvious when N≥11. There exists a reasonable value for the column spacing d to optimize the optical accuracy. In the example, the best parameters are d=950 mm and N=11, and the maximum deformation of the mirror surface is only 2.38 mm under 360 Pa wind pressure and self-weight. The errors of mirror slope D_x and D_y are only 5.31 mrad and 0.59 mrad. The structural deformations of the frame are only 0.72 mrad and 0.17 mrad to the D_x and D_y of the reflecting mirror surface. The structural stiffness of the frame is relatively affluent, and the stiffness of the reflecting mirror surface itself is the main reason for the degradation of its optical accuracy in service. The edge length a of the angle profiles in the rack between 30 mm and 50 mm has less effect on the service optical accuracy, while the effect of the thickness t is more significant. At t=4.0 mm, the service optical performance of a=30 mm and a=50 mm are comparable, and the former can reduce 1302 tons of steel in the case of the 50 MW solar tower power plant in Delingha, Qinghai. The preferred structural parameters are d=950 mm, N=11, H=380 mm, L=1225 mm, a=30 mm, and t=3.0 mm. Under different combinations of wind pressures (0-360 Pa) and height angles (0°, 45°, and 90°), the deformation of the structure and the error of the slope of the mirror surface increase linearly with the increase in the wind pressures, and the maximum deformation is in the range of 3.63-11.76 mm. While the slopes of the mirror total slope error change curves are basically the same for different height angles β, with only translational differences. When β=90°, the slope of the curve obtained by fitting the total mirror slope error with wind pressure is 0.0126, and the intercept is 1.439. Under the action of self-weight load only, the total slope error is in the range of 1.27-1.45 mrad for the height angles of 0°-90°. The distribution pattern of mirror deformation and its slope error is completely different. The relationship between mirror slope error and mirror deformation is not linear and proportional. In engineering practice, if the optimization design or evaluation of a heliostat structure is constrained by mirror deformation, satisfactory optical accuracy may not be obtained under a service load. Therefore, optical accuracy should be used to evaluate and guide the optimization design of the heliostat structure.